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TROPICAL STORM ISELLE DISCUSSION NUMBER 17
NATIONAL WEATHER SERVICE MIAMI FL
8 PM PDT WED SEP 18 2002
A SMALL BURST OF DEEP CONVECTION HAS DEVELOPED OVER AND TO THE WEST
OF THE LOW-LEVEL CENTER DURING THE PAST 4 HOURS. THEREFORE...ISELLE
IS MAINTAINED AS A MINIMAL TROPICAL STORM. THIS INTENSITY IS ALSO
SUPPORTED BY A DVORAK CURRENT INTENSITY ESTIMATE OF 35 KT FROM TAFB.
THE INITIAL MOTION ESTIMATE IS 050/03. ISELLE HAS BEEN DRIFTING
SLOWLY TO THE NORTHEAST OR EAST-NORTHEAST DURING THE PAST 6 HOURS.
NOW THAT DEEP CONVECTION HAS RE-DEVELOPED...THE CIRCULATION WILL
LIKELY BECOME MORE VERTICALLY DEEP AND BE STEERED SLOWLY EASTWARD BY
A WEAK MID- TO UPPER-LEVEL TROUGH AS SEEN WATER VAPOR IMAGERY. JUST
HOW FAR EAST IS THE BIG QUESTION. THE GFDL TAKES ISELLE EASTWARD TO
NEAR THE EXTREME WESTERN COAST OF THE BAJA PENINSULA IN 36 HOURS AND
THEN MOVES IT NORTHWARD ALONG THE COAST THROUGH 48 HOURS BEFORE
DISSIPATING IT IN 60 HOURS. THE REMAINDER OF THE NHC MODEL GUIDANCE
IS DIVERGENT ABOUT A GENERAL NORTH TO NORTHWESTWARD MOTION FROM THE
CURRENT POSITION. SINCE WATER VAPOR IMAGERY INDICATES AT LEAST 10
KT WESTERLY WINDS TO THE WEST OF ISELLE...THE TRACK WAS SHIFTED A
LITTLE TO THE RIGHT OR EAST OF THE PREVIOUS FORECAST. IF DEEP
CONVECTION PERSISTS OVERNIGHT...THEN THE TRACK MAY HAVE TO BE
SHIFTED EVEN FARTHER TO THE EAST ON THE NEXT ADVISORY AND TROPICAL
STORM WATCHES OR WARNINGS MAY BE REQUIRED FOR PORTIONS OF SOUTHWEST
BAJA. THE OFFICIAL TRACK TAKES ISELLE SLOWLY NORTHEAST THROUGH 12
HOURS AND THEN NORTH AND NORTHWESTWARD AFTER THAT. THIS IS BETWEEN
THE AVN-UKMET CONSENSUS AND THE GFDL-NOGAPS MODEL CONSENSUS...AND
KEEPS ISELLE AWAY FROM THE BAJA PENINSULA.
WATER VAPOR IMAGERY SUGGESTS THAT THE RECENT BURST OF CONVECTION IS
LIKELY THE RESULT OF A NARROW BAND OF MID- AND HIGH-LEVEL MOISTURE
ADVECTING ACROSS THE CYCLONE FROM THE WEST. SST ANALYSES INDICATE
THAT WARMER WATER...UP TO 28C...LIES BETWEEN THE CYCLONE AND THE
BAJA PENINSULA. THE SHEAR IS EXPECTED TO REMAIN LESS THAN 15 KT SO
SOME SLIGHT STRENGTHENING IS POSSIBLE. HOWEVER...THE MOISTURE BAND
TO THE WEST IS VERY NARROW...SO I HAVE DECIDED TO WAIT UNTIL THE
NEXT ADVISORY TO SEE IF AN INCREASE IN THE INTENSITY IS REQUIRED.
FORECASTER STEWART
FORECAST POSITIONS AND MAX WINDS
INITIAL 19/0300Z 22.8N 113.6W 35 KTS
12HR VT 19/1200Z 22.9N 113.2W 35 KTS
24HR VT 20/0000Z 23.3N 113.2W 30 KTS
36HR VT 20/1200Z 24.0N 114.0W 25 KTS...DISSIPATING
48HR VT 21/0000Z 25.0N 114.7W 20 KTS...REMNANT LOW
72HR VT 22/0000Z 26.0N 115.5W 20 KTS...REMNANT LOW
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